HB-EGF is synthesized as a membrane-anchored juxtacrine growth factor that is shred to released mature HB-EGF, a potent mitogen and chemotactic factor for smooth muscle cells (SMC). HB-EGF activity is mediated by two receptors, ErbB1 and ErbB4. For the most part HB-EGF function in vivo is not well characterized. In the first aim the role of SMC-derived HB-EGF in mediating SMC-EC interactions in blood vessels will be examined in vivo and in vitro. In addition, potential differences in the roles of transmembrane and mature HB-EGF will be analyzed in vivo in transgenic mice. The second aim involves characterization of novel HB-EGF receptors. One is a 140 kDa protein that has been recently purified and cloned, binds HB-EGF specifically and as a soluble receptor is a specific HB-EGF antagonist. In addition, two novel ErbB4 isoforms, one with an alteration in the juxtamembrane domain that can not be shed and one that lacks the PI3-kinase binding site and can not activate PI3-kinase activity will be further characterized. These proposed studies on HB-EGF function and receptors are significant since HB-EGF has been suggested to contribute to normal physiological responses such as wound healing and pathological processes such as atherosclerosis and pulmonary hypertension. The Specific Aims of the proposal are: 1. To Investigate the Role of HB-EGF in Blood Vessels including: a) analysis of temporal and spatial HB-EGF promoter-lacZ reporter gene expression in blood vessels of transgenic mice; b) analysis of the effects of HB-EGF on EC-SMC interactions in vitro; c) over- expression of mature, transmembrane and non-cleavable transmembrane forms in the blood vessels of transgenic mice; d) generation of transgenic mice expressing mature HB-EGF only, by deleting the transmembrane and cytoplasmic domains. 2. To Characterize Novel HB-EGF Receptors including: a) Structure and functional analysis of a novel specific 140 kDa HB-EGF receptor; b) characterization of novel alternatively spliced ErbB4 isoforms differing in the juxtamembrane domain and the PI-3K binding domains.